The invention concerns a conditioning device to change the moisture content of printing stock, especially flat paper products, in which the stock is brought into contact with at least one endless cloth that absorbs moisture, the endless cloth being heated by heated transport rolls.
EP 0 771 904 concerns a drying device in paper production, in which large amounts of moisture must be removed from the paper. In this case, a paper web is guided past a cloth that absorbs moisture, for example, made of felt, and exposed to infrared radiation in a limited region. At the same time, the moisture emerging from the paper web is drawn into the felt cloth by means of an underpressure from the bottom of the cloth, which contributes to an improvement in drying.
EP 0 078 382 concerns a drying device in offset printing. Infrared emitters are used to evaporate the moisture from paper sheets and a blower is used to blow out the moist air from the paper sheets and, at the same time, cool them.
The moisture content of paper is an important parameter that determines the dimensions of a paper sheet or paper web and therefore has a not insignificant effect on printing quality within a printer. The application of pressure and heat can cause shrinkage of paper stock. This has a particular effect during double-sided printing, if a change in moisture content of the stock occurs between printing of the front and backsides, and a change in size of the stock is caused by this. This produces so-called front-to-back error. This is a regular occurrence in digital printers that use heat and pressure to fix a toner image. This effect can be reduced by predrying of the stock, since only limited shrinkage can still occur, because of the more limited residual moisture content of the stock. An increase in printing quality is achieved on this account.
It is therefore the task of this invention to provide an apparatus, in which it is guaranteed that the stock reaches the subsequent processing steps of the printer with a defined geometry and a defined moisture content.
In an advantageous embodiment of the concept underlying the invention, a printing stock is brought in contact with at least one endless cloth that absorbs moisture, which is stretched around a number of transport rolls that serve for movement of the endless cloth, and heated by means of a heating device, so that the moisture within the printing stock also heated by this is transferred to the endless cloth.
In a particularly advantageous variant, the heating device lies within the rolls and heats them from the inside. This avoids a direct effect of the heating device on the printing stock, which can lead to changes, for example, in the printing image or color composition and, in so doing, cause deterioration in printing quality.
Printing stock that comes in contact with the endless cloth heated by the heating device is heated and releases its moisture content to the surroundings, and especially to the endless cloth. The moisture content is further transported within the endless cloth and removed from the cloth at an appropriate site. This is supported by additional contact of the endless cloth with the heated transport rolls, which contribute to evaporation of the moisture from the endless cloth.
In addition to evaporation of the moisture from the endless cloth, an air stream can be generated in a modification of the concept of the invention, especially by a blower, which has at least one flow component directed horizontal to the surface of the endless cloth and is thus guided past the endless cloth. Evaporation of the moisture from the endless cloth is supported by the change in local vapor pressure caused by this. This process can be preferably supported according to the invention in that the endless cloth is dried outside of the contact zone.
In another variant, an air stream generated, in particular, by a blower and having at least one flow component directed perpendicular to the surface of the endless cloth can be guided through the endless cloth and entrain moisture particles, which also contributes to acceleration of drying of the endless cloth. According to the invention, the air stream that is guided through the endless cloth or past the endless cloth will have a temperature deviating from the surrounding temperature, especially an elevated temperature.
In the region of the conditioning device, the paper is advantageously conveyed forward by the mechanical contact between the endless cloth and the printing stock. Alternative methods of conveyance, having a drive on the side of the printing stock facing away from the endless cloth, are also conceivable.
In another advantageous embodiment of the device according to the invention, the printing stock is guided between two identical endless cloths arranged on both sides of the printing stock. This can guarantee a more uniform removal of moisture from the printing stock, since the evaporated moisture here is fully transferred into the endless cloths.
A significant advantage of this embodiment according to the invention is obtained by the fact that costly paper guides drop out. At the same time, relative movement between the printing stock and the endless cloth is reduced by the forward movement, which is transferred by the endless cloths to the printing stock, and contact with stationary paper guides prevented. Because of this, a situation is avoided in which the surface of the printing stock is altered, especially damaged.
It lies within the inventive concept of the device disclosed here that heating of the transport rolls of the endless cloth need not occur uniformly over their width, but can be superimposed with a temperature profile. A variation of the temperature profile in the longitudinal direction can also be simultaneously implemented, in which the power of the heating devices is varied in time, especially in correlation with the format of the printing stock.
In another advantageous variant, an evaporation zone is connected to the tempering zone, in which the printing stock releases moisture, because of its heating, and is cooled based on the released heat of evaporation. Advantageously, guiding of the printing stock can occur in this region through grid-like structures that afford maximum evaporation surface.
In another variant, a cooling device can advantageously be mounted behind the tempering and/or evaporation zone. However, unheated or cooled transport rolls are used. Because of this, a situation can be achieved in which the printing stock, after leaving the cooling device, has essentially the same temperature as before the drying device. It lies within the scope of the concept according to the invention presented here to use the cooling device also for other purposes without a preceding tempering zone, especially for cooling of the printing stock.
Another advantage is obtained from the elasticity, especially compressibility of the endless cloths that absorb the moisture, so that thickness differences of individual types of printing stock are compensated and no corresponding adjustments are therefore necessary or excess loading of the rolls by different thicknesses of the printing stocks can be prevented. By contact of the printing stock with the endless cloth and removal of the evaporated moisture connected with this from the printing stock, traces on the printing stock, like, water spots, are also avoided.
Printing stock is sometimes deformed, especially rolled up, so-called paper curl, by storage or the production process. A further advantage of the device according to the invention lies in the fact that deformed printing stock is kept flat and smooth by close contact with the endless cloths.
Electromagnetic radiation sources, especially infrared radiation sources, can be advantageously used as heating devices to heat the transport rolls. However, other principles for heating the transport rolls are equally conceivable, like, steam, electric heating or others.
Blowers or air nozzles of a wide variety of arrangements and methods of operation can be used as blower devices.
The device according to the invention finds application in digital printers, but can also be used in all other printing machines or other devices that process paper where differences in paper size during passage through the device and its components are to be avoided or at least reduced.